1. Technical Field
The present invention relates to an improvement on a transit-time difference type (referred hereinafter to as time difference type) ultrasonic flowmeter.
2. Description of the Prior Art
The arrangement of the transit-time difference type ultrasonic flowmeter of the prior art will now be described with reference to FIG. 6.
A pair of ultrasonic transducers 2a and 2b are provided on both ends of a straight section 1a of a flow tube 1 including a fluid inlet 1b and a fluid outlet 1c extending vertically from the ends of the straight section 1a. 
A switching device 3 is provided between the ultrasonic transducers 2a and 2b and an excitation pulse generator 4 and an amplifier 5 receiving the signal from one of the transducers. The switching device 3 delivers pulses from the excitation pulse generator 4 to the one of the transducers and receives signals from the other of the transducers and delivers the signals to the amplifier 5 vise versa.
The signal for causing the switching is not described herein.
A comparator 6 detects the time (the received time) on which the received waveform amplified through the amplifier 5 exceeds the reference voltage provided by a source of reference voltage 7.
A data reduction equipment 8 receives the output from the comparator 6, calculates the time duration from the excitation time to the received time, in other words the ultrasonic wave propagating time i.e. the time required for propagating the ultrasonic wave generated by one of the ultrasonic transducers through fluid to the other transducer, and output a flow rate signal obtained by effecting the following equation.
Tuxe2x88x92Td=2LV/C2 
∴V=(Tuxe2x88x92Td)C2/2L 
Q=SV 
wherein Tu is a time for propagating the ultrasonic wave upstream-wards, Td is a time for propagating the ultrasonic wave downstream-wards, V is a flow velocity through the flow tube, Q is a flow rate through the flow tube, C is acoustic velocity, and L is a distance between the transducers.
However, the following technical problems are still present in the time difference type ultrasonic flowmeter of the prior art.
The comparator cannot distinguish the received signal from noise superposed on it when the flowmeter is operated under the circumstance flooded by electric pulse noise. In such a case, the measured value of the propagation time will inevitably be fluctuated and the affect caused thereby will be augmented when the flowmeter is used for measuring relatively low flow rate.
Further, provided that impurities such as bubbles or solid particles are included in the fluid, the amplitude of the received signal is tend to attenuate, and sometimes makes the result of the measurement unstable.
As shown in FIG. 7, the received waveform with no interference is illustrated by a solid line, the waveform attenuated under the effect of impurities such as bubbles or solid particles is illustrated by a broken line, and the reference voltage Vc of the comparator is illustrated by a dashed line. As can be seen from FIG. 7, the waveform to be measured is varied under the effect of the attenuation, so that the consistent measurement cannot be effected since the propagating time is measured at Ta or Tb.
In order to solve the above mentioned problem, the first transit-time difference type ultrasonic flowmeter of the present invention comprises:
a pair of ultrasonic transducers mounted on an outer surface of a flow tube at an upstream side and a downstream side respectively, a switching device for switching the operational mode of each of the pair of transducers alternatively to its transmitting or receiving mode, an amplifier for amplifying the signal representing the ultrasonic waves propagating through the fluid received by the ultrasonic transducer of the receiving side, and a data reduction equipment for processing the amplified received signal to output a flow rate signal;
the data reduction equipment includes an analog digital converter and a digital signal processor;
the analog-digital converter converts the waveform of the received signal amplified by the amplifier into a plurality of voltage-time data sets; and
the digital signal processor picks up on the basis of the data of time an object peak from the waveform of the voltage-time data sets, estimates at least one zero-cross point confined by the object peak or a peak adjacent to the object peak through the calculation made on the plurality of voltage-time data sets distributing along the time axis close to the zero-cross points, and finds the time required for propagating the ultrasonic wave from one zero-cross point or the average time required for propagating the ultrasonic wave from a plurality of zero-cross points. The first transit-time difference type ultrasonic flowmeter is characterized by the fact that the object peak is picked up from peaks present within the preset time interval.
The second transit-time difference type ultrasonic flowmeter of the present invention comprises:
a pair of ultrasonic transducers mounted on an outer surface of a flow tube at an upstream side and a downstream side respectively, a switching device for switching the operational mode of each of the pair of transducers alternatively to its transmitting or receiving mode, an amplifier for amplifying the signal representing the ultrasonic waves propagating through the fluid received by the ultrasonic transducer of the receiving side, and a data reduction equipment for processing the amplified received signal to output a flow rate signal;
the data reduction equipment includes an analog-digital converter and a digital signal processor;
the analog-digital converter converts the waveform of the received signal amplified by the amplifier into a plurality of voltage-time data sets; and
the digital signal processor picks up on the basis of the data on the value of voltage an object peak from the waveform of the voltage-time data sets, estimates at least one zero-cross point confined by the object peak or a peak adjacent to the object peak through the calculation made on the plurality of voltage-time data sets distributing along the time axis close to the zero-cross points, and finds the time required for propagating the ultrasonic wave from one zero-cross point or the average time required for propagating the ultrasonic wave from a plurality of zero-cross points. The second transit-time difference type ultrasonic flowmeter of the first embodiment is characterized by the fact that the object peak is picked up from peaks of the voltage over the predetermined value.
The third transit-time difference type ultrasonic flowmeter of the present invention comprises:
a pair of ultrasonic transducers mounted on an outer surface of a flow tube at an upstream side and a downstream side respectively, a switching device for switching the operational mode of each of the pair of transducers alternatively to its transmitting or receiving mode, an amplifier for amplifying the signal representing the ultrasonic waves propagating through the fluid received by the ultrasonic transducer of the receiving side, and a data reduction equipment for processing the amplified received signal to output a flow rate signal;
the data reduction equipment includes an analog-digital converter and a digital signal processor;
the analog-digital converter converts the waveform of the received signal amplified by the amplifier into a plurality of voltage-time data sets; and
the digital signal processor determines the peak of maximum voltage included within the waveform of the voltage-time data sets as an object peak, estimates at least one zero-cross point confined by the object peak or a peak adjacent to the object peak through the calculation made on the plurality of voltage-time data sets distributing along the time axis close to the zero-cross points, and finds the time required for propagating the ultrasonic wave from one zero-cross point or the average time required for propagating the ultrasonic wave from a plurality of zero-cross points.
According to the first embodiment, the digital signal processor measures the height of the object peak and estimates the amount of attenuation of the received signal due to the impurities such as bubbles or solid particles included in the fluid, wherein if the amount of attenuation is larger than the preset value, the calculation for estimating the zero-cross points is suspended, and the flow rate signal is output by adopting the measured value of time required for propagation or the output value of flow rate obtained on the last measurement of the repeatedly effected measurements.
According to the second embodiment, the digital signal processor measures the height of the object peak, makes comparison between the height of the object peak when the ultrasonic wave propagates upstream-wards and the height of the object peak when the ultrasonic wave propagates downstream-wards, wherein if the difference between the height of the object peaks is larger than the predetermined value, the calculation for estimating the zero-cross points is suspended, and the flow rate signal is output by adopting the measured value of time required for propagation or the output value of flow rate obtained on the last measurement of the repeatedly effected measurements.
According to the third embodiment, the digital signal processor measures a ratio defined between the heights of the peaks before and after the zero-cross point which is the object of the measurement of time required for propagation, if the difference between the obtained ratio of the heights of peaks and a ratio obtained upon measured on the fluid including no impurities such as bubbles or solid particles is larger than the predetermined value, the calculation for estimating the zero-cross points is suspended, and the flow rate signal is output by adopting the measured value of time required for propagation or the output value of flow rate obtained on the last measurement of the repeatedly effected measurements.
According to the fourth embodiment, the digital signal processor is adapted to alter according to the variation of the heights of the object peaks the number of zero-cross points defining the base on which the mean value of the time required for propagation is calculated.
According to the fifth embodiment, the digital signal processor processes the plurality of voltage-time data set through least squares method, calculate a regression line or curve, and estimates the time required for propagation of the zero-cross points.